Summary: Breast cancer is the leading cause of cancer-related death in women worldwide. Tumor-associated macrophages (TAMs) populate the tumor microenvironment (TME), adopt an alternative (M2) polarized phenotype, and play critical roles in cancer initiation, progression and metastasis. Emerging data suggests that M? polarization is accompanied by metabolic reprogramming. During M2 polarization, M?s obtain much of their energy from fatty acid oxidation and oxidative phosphorylation. M?s take up low-density lipoprotein (LDL) through LDL receptor (LDLR)-mediated endocytosis. These LDL particles provide M?s with fatty acids through hydrolysis of their phospholipids, triglycerides and cholesterol ester. Therefore, LDLR-mediated intracellular LDL accumulation may render M?s tumor-promoting. The discovery of PCSK9 merely a decade ago and ensuing genetic studies have highlighted this protein as an attractive therapeutic target for reducing cardiovascular mortality. Inhibition of PCSK9 increases hepatocyte LDLR levels, resulting in reduction of plasma LDL-c. However, the impact on tissue M?s including TAMs has not been investigated. Because M? LDLR expression should be tightly controlled in order not to overload M?s with LDL, PCSK9 inhibition may significantly increase M? LDLR levels and intracellular lipid accumulation, and thereby affect the function of M?s. With the FDA approval of the PCSK9 monoclonal antibodies alirocumab and evolocumab, and hundreds of thousands of patients have since been or will be on PCSK9 inhibition treatment, a concern is that long-term inhibition of PCSK9, while achieving the goal of reducing cardiovascular disease, may compromise M? function and drive malignant processes. Indeed, data from an earlier study indicated that in breast cancer stroma, PCSK9 expression is significantly reduced while LDLR expression increased. We recently found that PCSK9 is expressed in M?s, and regulates their surface LDLR levels and their function. Furthermore, our preliminary data show that conditioned medium from breast cancer cells reduces PCSK9 expression in M?s, and that PCSK9 deficient M?s adopt a more M2-like (pro-tumor) phenotype under the treatment of breast cancer cell conditioned medium. These data suggest that M? PCSK9 expression may be modulated by and impact the TME in breast cancer. To further this line of study, we will test our central hypothesis that lack of PCSK9 may prime M?s to a pro-tumor phenotype and thus promote breast cancer development. Two specific aims are proposed. SA1. To examine the impact of M? PCSK9 deficiency on breast cancer growth and metastasis. SA2. To examine the effects of PCSK9 deficiency on M? function in the context of breast cancer and determine the underlying mechanisms. The proposed investigation is directly responsive to Provocative Question 3: How do variations in tumor-associated immune responses contribute to differences in cancer risk, incidence, or progression? If the central hypothesis is proven, it will alert clinicians that female patients treated with PCSK9 inhibitors may have increased risk for breast cancer due to altered M? function.